Field of the Invention
Embodiments of the present invention relate to an electrophotographic image forming apparatus, such as a copier, a printer, or a facsimile apparatus.
Description of the Related Art
An image forming apparatus may be constituted by an image forming apparatus main body and a developing cartridge to which a developing device is detachably mounted, or may be constituted by an image forming apparatus main body and a process cartridge in which a developing device, an image bearing member, and other image forming process units are integrated.
A charging roller is widely used as a charging unit that provides a desired potential to an image bearing member. The charging roller is brought into contact with the image bearing member and charges the surface of the image bearing member through a discharge while rotating.
In addition, according to a known technique for removing developer that remains after a developer image formed on an image bearing member is transferred onto a recording material, the developer is removed by causing a cleaning blade to make contact with the image bearing member in the direction counter to the direction in which the image bearing member rotates.
The charging roller that has made contact with the image bearing member may experience a charging failure as toner or an external additive missed by the cleaning blade adheres to the charging roller, and an image of a vertical streak or the like may be generated. Thus, according to Japanese Patent Laid-Open No. 2002-311692, in order to remove dirt that has adhered to a charging roller, the charging bias is switched while an image is not formed, and thus dirt on the charging roller is removed.
According to Japanese Patent Laid-Open No. 04-371972, the number of times the charging bias for removing dirt is applied is changed in accordance with the extent to which a process cartridge has been used.
According to Japanese Patent Laid-Open No. 2003-280335, the control of the charging bias for removing dirt is changed when the cumulative print ratio has exceeded a threshold value set in advance.
According to Japanese Patent Laid-Open No. 2000-029281, the control of the transfer bias for removing dirt on a transfer unit is changed in accordance with the number of successively printed sheets.
However, along with the reduction in the size of the process cartridge in recent years, the diameter of the charging roller is being reduced. Thus, the number of rotations of the charging roller has increased due to the decrease in the diameter of the charging roller, and the frequency at which the charging roller makes contact with an image bearing member has increased, which has led to a situation in which dirt is more likely to accumulate on the charging roller. Furthermore, when images are to be formed successively, adhered matter on the charging roller accumulates.
Therefore, a removing voltage is applied to the charging roller during a non-image-forming operation so as to remove the adhered matter. However, if the control is carried out in accordance with the print ratio of image formation, an external additive of polarity that is opposite to the polarity of the toner is supplied even for a solid white image, and thus the external additive is likely to adhere to the charging roller.
In addition, if the control is carried out in accordance with the number of successively printed sheets, the cleaning blade may miss the toner or the external additive near the cleaning blade, or the removing control may be executed more than necessary.